A Zener diode is a solid state device having two contiguous regions of opposite conductivity type (P-type and N-type) and a PN junction at the interface of the two regions. Typically, one of the regions is more lightly doped than the other, and is the region chiefly affecting breakdown voltage of the PN junction.
One of the most common uses of series-wired light strings is for decoration and display purposes, particularly during Christmas and other holidays, and more particularly for the decoration of Christmas trees, and the like. Probably the most popular light set currently available on the market, and in widespread use, comprises one or more strings of fifty miniature light bulbs each, with each bulb typically having an operating voltage rating of 2.5 volts, and whose filaments are connected in a series circuit arrangement. If overall sets of more than fifty bulbs are desired, the common practice is to provide a plurality of fifty miniature bulb strings, with the bulbs in each string connected in series, and with the plurality of strings being connected in a parallel circuit arrangement.
When a string of bulbs is connected in series, if a single bulb fails to illuminate for any reason, the whole string fails to light and it is very frustrating and time consuming to locate and replace a defective bulb or bulbs. Usually many bulbs have to be checked before finding the failed bulb. In fact, in many instances, the frustration and time consuming efforts are so great as to cause one to completely discard and replace the string with a new string before they are even used. The problem is compounded when multiple bulbs simultaneously fail to illuminate for multiple reasons, such as, for example, one or more faulty light bulbs, one or more unstable socket connections, or one or more light bulbs physically fall from their respective sockets, and the like.
There are presently available on the market place various devices and apparatuses for electrically testing an individual light bulb after it has been physically removed from its socket. An apparatus is also available on the market for testing Christmas tree light bulbs by physically placing an alternating current line voltage sensor in close proximity to the particular light bulb desired to be tested. However, such a device is merely an electromagnetic field strength detection device, which may remain in an “on” condition whenever the particular Christmas tree light bulb desired to be tested is physically located in close proximity to another light bulb or bulbs on the Christmas tree.
Light bulb manufacturers have also attempted to solve the problem of detecting a failed bulb by designing each light bulb in the string in a manner whereby the filament in each light bulb is shorted whenever it burns out, thereby preventing an open circuit condition to be present in the socket of the burned-out bulb. However, in actual practice, it has been found that such short circuiting feature within the bulb does not always operate in the manner intended and the entire string will go out whenever a single bulb burns out.
U.S. Pat. No. 5,539,317, entitled CIRCUIT TESTER FOR CHRISTMAS TREE LIGHT SETS, issued Jul. 23, 1996, discloses a hand held and battery operated device which is capable of testing each light bulb in a string without the necessity of removing the bulb from its socket, thereby readily locating the burned out bulb which caused the entire string of bulbs to go out.
Even though each of the foregoing techniques have met with some limited success, none of such devices and techniques have yet been able to further solve the additional problems of the entire string of lights going out as a direct result of either a defective socket, a light bulb being improperly placed in the socket, a broken or bent wire of a light bulb, or whenever a light bulb is either intentionally removed from its socket or is merely dislodged from its socket during handling or movement after being strung on the Christmas tree, particularly in outdoor installations which are subject to wind or other climatic conditions.
U.S. Pat. No. 4,450,382 utilizes a Zener diode connected in parallel with each series connected direct-current lamp used by trucks and other vehicles, particularly military trailers, for burn-out protection of the remaining bulbs whenever one or more bulbs burns out for some reason. It is stated therein that the use of either a single or a plurality of parallel connected Zener diodes will not protect the lamps against normal failure caused by normal current flows, but will protect against failures due to excessive current surges associated with the failure of associated lamps. No suggestion appears therein of any mechanism or technique which would provide a solution to the problem in a simple and economical manner. A plurality of Zener diodes is used to distribute the heat that would be dissipated from a headlight situation where large currents are employed in such a lighting system. The invention does not suggest to use a single high wattage Zener because all of the heat that would be generated would be in one area and cooling would be a problem. Therefore, by using a plurality of lower wattage Zener diodes, the heat could be distributed easier.
Various other attempts have been made to provide various types of shunts in parallel with the filament of each bulb, whereby the string will continue to be illuminated whenever a bulb has burned out, or otherwise provide an open circuit condition, which are commercially feasible.
Examples of such arrangements are found in U.S. Pat. Nos. Re 34,717; 1,024,495; 2,072,337; 2,760,120; 3,639,805; 3,912,966; 4,450,382; 4,682,079; 4,727,449; 5,379,214; 5,006,724, 6,580,182, together with Great Britain Patent No. 12,398; Swiss Patent No. 427,021 and French Patent No. 884,370.
Some of these prior art patents provide for continued current flow through the string, but at either a higher or a lower level. The reason for this is that the voltage drop occurring across each prior art shunt is substantially different value than the value of the voltage drop across the incandescent bulb during normal operation thereof.
Several prior art shunts cause an increase in current flow in the series string as a result of a decrease in the voltage dropped across the socket. The higher voltage applied to the remaining bulbs results in higher current flow and a decreased life expectancy of the remaining bulbs in the string. Additionally, the higher voltage also results in increased light output from each of the remaining bulbs in the string, which may not be desirable in some instances.
However, other shunt devices cause the opposite effect, namely an undesired reduced current flow because a high voltage drop occurs across the shunt when a bulb becomes inoperable, either due to an open filament, a faulty bulb, a faulty socket, simply because the bulb is not mounted properly in the socket, or is entirely removed or falls from its respective socket. A reduced current flow results in a corresponding decrease in light output from each of the remaining bulbs in the string. Such an undesirable effect occurs in the prior art attempts, especially the proposed use of a diode in series with a bilateral switch in the Fleck '449 patent, or the proposed use of a metal oxide varistor in the above Harnden '966 patent, or the use of the proposed counter-connected rectifiers in the Swiss '021 patent.
For example, in the arrangement suggested in the above Fleck '449 patent, ten halogen filled bulbs, each having a minimum 12-volt operating rating, are utilized in a series circuit. The existence of a halogen gas in the envelope permits a higher value current flow through the filament allowing for much brighter light to be obtained in a very small bulb size. Normally, when ten 12-volt halogen bulbs are connected in a series string, the whole string goes dark whenever a single bulb fails and does not indicate which bulb has failed. To remedy this undesirable effect, Fleck provided a bypass circuit across each halogen filled bulb that is comprised of a silicon bilateral voltage triggered switch in series with a diode which rectifies the alternating current (i.e., “A.C.”) supply voltage and thereby permits current to flow through the bilateral switch only half of the time, i.e., only during each half cycle of the A.C. supply voltage. As stated in Fleck, when a single bulb burns out, the remaining bulbs will have “diminished” light output because the diode will almost halve the effective voltage due to its blocking flow in one direction and conduction flow only in the opposite direction. Such a substantially diminished light output will quite obviously call attention to the failed bulb, as well as avoid the application of a greater voltage which would decrease the life of the remaining filaments. However, in actual practice, a drastic drop in brightness has been observed, i.e. a drop from approximately 314 lux to approximately 15 lux when one bulb goes out. Moreover, as is stated in the patent, the procedure of replacing a burned out bulb involves the interruption of the application of the voltage source in order to allow the switch to open and to resume normal operation after the bulb has been replaced. Additionally, as such an arrangement does not permit more that one bulb to be out at the same time, certain additional desirable special effects such as “twinkling”, and the like, would not be possible.
In the arrangement suggested in Harnden '966 patent, Harden proposes to utilize a polycrystalline metal oxide varistor as the shunting device, notwithstanding the fact that it is well known that metal oxide varistors are not designed to handle continuous current flow therethrough. A metal oxide varistor is merely a so-called “one shot” device for protective purposes, i.e. a transient voltage suppressor that is intended to absorb high frequency or rapid voltage spikes and thereby preventing such voltage spikes from doing damage to associated circuitry. Metal oxide varistors are designed for use as spike absorbers and are not designed to function as a voltage regulator or as a steady state current dissipation circuit. While metal oxide varistors may appear in some cases similar to back-to-back Zener diodes, they are not interchangeable and function very differently according to their particular use. In fact, the Harris Handbook states in Application Note 9311: “They are exceptional at dissipating transient voltage spikes but they cannot dissipate continuous low level power.” The Harris Handbook further states that its metal oxide varistors cannot be used as voltage regulators as their function is to be used as a nonlinear impedance device. The only similarity that one can draw from metal oxide varistors and back-to-back Zener diodes is that they are both bidirectional.
In the Swiss '021 patent, Dyre discloses a bilateral shunt device having a breakdown voltage rating that, when exceeded, lowers the resistance thereof to 1 ohm or less. This low value of resistance results in a substantial increase in the voltage applied to the remaining bulbs, even when only a single bulb is inoperative for any of the reasons previously stated. Thus, when multiple bulbs are inoperative, an even greater voltage is applied to the remaining bulbs, thereby again substantially increasing their illumination, and consequently, substantially shortening their life expectancy.
In contrast, by utilizing a shunt of the type proposed in the present invention, a substantial number of the bulbs in a 50 bulb string can become inoperative for any or all of the reasons previously stated, with a minimal decrease in intensity of illumination of the remaining bulbs, which is not possible with any of the foregoing shunts. In fact, miniature Christmas tree type lights now rely solely upon a specially designed bulb which is intended to short out upon becoming inoperative. However, such a scheme is not always effective, particularly when a bulb is removed from its socket or becomes damaged in handling, etc. An attempt made by others to keep the bulbs from falling from their sockets is the use of a locking groove formed on the inside circumference of the socket mating with a corresponding raised ridge formed on the base of the bulb base unit. While this particular locking technique apparently is very effective to keep bulbs from falling from their respective sockets, the replacement of defective bulbs by the average user is extremely difficult, if not sometimes impossible, without resorting to mechanical gripping devices which can actually destroy the bulb base unit or socket.
The arrangement of the series-wired light string of the present invention and its function is disclosed and described in U.S. Pat. Nos. 6,580,182 and 6,597,125, which are incorporated herein in their entirety by reference.